Official Explanation:

(A) The activist has done two things that need to be made parallel—railed against the callousness and the negligence and tried to forge something. Hence, there needs to be an and between callousness and negligence.

(B) The use of the infinitive to rail after the comma is incorrect. Also, to rail is not parallel to trying.

(C) This construction tries to make three things parallel – has spent, railed, and tried. This is obviously incorrect as the sentence does not try to convey this. Also, the and is missing before negligence.

(D) The correct answer.

(E) The and is missing before neglecting. Also, the use of neglecting changes the meaning of the sentence by suggesting that the upper classes are being neglected.
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Answer is D. Correct parallelism. Railing modifies the activist.

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The actual sentence :
A vocal opponent of the unequal distribution of wealth, the itinerant activist has spent time in several countries across the world,
railing against the callousness and injustices of governments,
the negligence of the upper classes, and
trying to forge a transnational people’s movement.

The activist has spent time -- the subsequent part of the sentence describes HOW has the activist spent the time.
Thus, we need, VERB-ING participle modifier to describe the clause and apply it to the subject "the activist".

Let's look at the choices.
(A) railing against the callousness and injustices of governments, the negligence of the upper classes, and trying to forge a transnational people’s movement
Railing against... and trying to... are two VERB+ing modifiers. "The negligence" is a noun.
Note : A noun phrase could have been correct only if it were modifying the previous noun or noun phrase.
Here, the callousness and injustices of governments has nothing to do with the negligence of the upper classes. .
Thus, "the negligence" does not fit here.
(B) to rail against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement
The infinitive " to rail" implies that the activist spent the time purposefully. It's not intended. "To rail" and "trying" are not parallel.
(C) railed against the callousness and injustices of governments, the negligence of the upper classes, and tried to forge a transnational people’s movement
VERB+ED modifiers (after main clause) don't describe the HOW aspect of the clause. We need VERB+ING modifier.
Also, a noun phrase could have been correct only if it were modifying the previous noun or noun phrase.
Here, the callousness and injustices of governments has nothing to do with the negligence of the upper classes. .
Thus, "the negligence" does not fit here.
(D) railing against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement
The activist has spent the time, railing against X of government and Y of upper classes, and trying to do Z.
The VERB+ING modifier correctly modifies the main clause, describing HOW has he spent the time.
CORRECT
(E) railing against the callousness and injustices of governments, the neglecting of the upper classes, and trying to forge a transnational people’s movement
"the neglecting of the upper classes" is a noun phrase . As explained in A and C, It does not fit with VERB+ING modifiers.

D

My answer is (D). It took me 01:40. Strictly speaking, the last comma is (D) should better be removed, but I do agree that the current version in (D) is clearer.

In any test, we actually prefer strict rules as they can help us quickly eliminate incorrect options.
If we want to "AND" two items together, we write: X and Y. We do not write: X, and Y. Neither do we write: X, Y
If we want to "AND" three+ items together (to form a list), we write: X, Y, and Z.

(A) "railing against" is parallel with "trying to forge". Two items (the callousness and injustices of governments AND the negligence of the upper classes) are used as object of "railing against". For two items, we cannot write X, Y. (A) is eliminated.
How about the comma before "and trying"? Strictly speaking, it is not needed either

(B) This option does fix the identified issue with the object of "rail against". It contains other problems though.
b.1 No parallelism between "to rail against" (infinitive) and "trying to" (present participle).
b.2 If infinitive is used, I do not think we need to set it aside with the comma before "to rail against'.
b.3 If we do remove the comma, it also breaks the idiomatic use of "spend time / money (in) doing something".

(C) We can find parallelism between "spent", "railed against", and "tried to forge". But, two items (the callousness and injustices of governments AND the negligence of the upper classes) are used as object of "railed against". For two items, we cannot write X, Y.
Moreover, "rail against" and "try to forget" are used to illustrate what the activist spent time doing. They are not meant to be of equal with "spend time". So, it makes more sense to use participles.

(D) Compare this option with (A). It fixes the first issue: Two items (the callousness and injustices of governments AND the negligence of the upper classes) are used as object of "railing against". (D) correctly uses A and B.
How about the second issue? Strictly speaking, the last comma is (D) should better be removed, but I do agree that the current version in (D) is clearer.

(E) Eliminated as it basically duplicates issues in (A).

A vocal opponent of the unequal distribution of wealth, the itinerant activist has spent time in several countries across the world, railing against the callousness and injustices of governments, the negligence of the upper classes, and trying to forge a transnational people’s movement.


(A) railing against the callousness and injustices of governments, the negligence of the upper classes, and trying to forge a transnational people’s movement -> We have list items "railing", " the negligence" and "trying" are not parallel. Incorrect.

(B) to rail against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement -> We need -ing modifier to start with. Incorrect.

(C) railed against the callousness and injustices of governments, the negligence of the upper classes, and tried to forge a transnational people’s movement -> We need -ing modifier to start with. Incorrect.

(D) railing against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement -> We have list items "railing", and "trying" are parallel. Let's keep it.

(E) railing against the callousness and injustices of governments, the neglecting of the upper classes, and trying to forge a transnational people’s movement -> We have list items "railing", " the neglecting" and "trying" are parallel. But, meaning wise, "the neglecting of the upper classes" is not a list item, rather "railing" is a list item. Incorrect.

So, I think D.

A vocal opponent of the unequal distribution of wealth, the itinerant activist has spent time in several countries across the world, railing against the callousness and injustices of governments, the negligence of the upper classes, and trying to forge a transnational people’s movement.


(A) railing against the callousness and injustices of governments, the negligence of the upper classes, and trying to forge a transnational people’s movement Incorrect

parallelism error - ....., railing ......., the negligence........, and trying ...........

(B) to rail against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement Incorrect

parallelism error - ....spent ......., to rail........, and trying .......

(C) railed against the callousness and injustices of governments, the negligence of the upper classes, and tried to forge a transnational people’s movement Incorrect

parallelism error - ....., railed ......., the negligence........, and tried ...........

(D) railing against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement Correct

(E) railing against the callousness and injustices of governments, the neglecting of the upper classes, and trying to forge a transnational people’s movement Incorrect

parallelism error - ....., railing ......., the neglecting........, and trying ...........

IMO D


A vocal opponent of the unequal distribution of wealth, the itinerant activist has spent time in several countries across the world, railing against the callousness and injustices of governments, the negligence of the upper classes, and trying to forge a transnational people’s movement.


(A) railing against the callousness and injustices of governments, the negligence of the upper classes, and trying to forge a transnational people’s movement

the way to add “the negligence of the upper classes” could be better.

If it is parallel to “callousness and injustices of governments” that the activists rail against. It should add “and” before the noun phrase.

The current structure makes it seem like it is parallel to “railing against...” and “trying to forge...”. Confusing.

(B) to rail against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement

”To rail against...” is not parallel to “trying to forge”
Eliminate.

(C) railed against the callousness and injustices of governments, the negligence of the upper classes, and tried to forge a transnational people’s movement
Similar problem as A. Eliminate both A and C.

(D) railing against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement
Correct

(E) railing against the callousness and injustices of governments, the neglecting of the upper classes, and trying to forge a transnational people’s movement
”the neglecting of upper classes” is now parallel to “railing against...” and “trying to forge” but the meaning is not right. Eliminate.

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A vocal opponent of the unequal distribution of wealth, the itinerant activist has spent time in several countries across the world, railing against the callousness and injustices of governments, the negligence of the upper classes, and trying to forge a transnational people’s movement.


(A) railing against the callousness and injustices of governments, the negligence of the upper classes, and trying to forge a transnational people’s movement

(B) to rail against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement

(C) railed against the callousness and injustices of governments, the negligence of the upper classes, and tried to forge a transnational people’s movement

(D) railing against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement

(E) railing against the callousness and injustices of governments, the neglecting of the upper classes, and trying to forge a transnational people’s movement

Choice D is the answer.

ACT: the itinerant activist has spent time in several countries across the world
Modifiers for above ACT:
1. railing against the callousness and injustices of governments
2. railing against the negligence of the upper classes
3. trying to forge a transnational people’s movement

Modifier shall be placed after subject + , modifier

Here we have multiple BACK to BACK modifiers, to be arranged in parallel.
Modifier 1 & 2 can be arranged directly with and without comma, common action 'railing against'
railing against the callousness and injustices of governments and the negligence of the upper classes

Modifier 3 is having differant action. So comma + and is required.

FINAL SENTENCE

A vocal opponent of the unequal distribution of wealth, the itinerant activist has spent time in several countries across the world, railing against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement.

Choice D
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A vocal opponent of the unequal distribution of wealth, the itinerant activist has spent time in several countries across the world, railing against the callousness and injustices of governments, the negligence of the upper classes, and trying to forge a transnational people’s movement.

Pre-thinking: There are 2 sets of parallelism in the above sentence - 'railing against' and 'trying to is one pair, 'the callousness' and 'the negligence' is another pair.

(A) railing against the callousness and injustices of governments, the negligence of the upper classes, and trying to forge a transnational people’s movement
and is missing between the parallel pair of 'the callousness' and 'the negligence' which is causing railing against', 'trying to, 'the callousness' and 'the negligence' to seem like a pair of 4 in the same parallel structure. which is incorrect.

(B) to rail against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement
'to rail' and 'and trying to' are not parallel.

(C) railed against the callousness and injustices of governments, the negligence of the upper classes, and tried to forge a transnational people’s movement
same error as Option A.

(D) railing against the callousness and injustices of governments and the negligence of the upper classes, and trying to forge a transnational people’s movement
Correct placement of and to distinguish between two pairs of parallel structures.

(E) railing against the callousness and injustices of governments, the neglecting of the upper classes, and trying to forge a transnational people’s movement
same error as Option A.

Answer should be Option D.
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I think comma before 'and trying' in option D is not correct. Request expert's views on this.